1. Field of the Invention
The present invention relates to a light-emitting array comprising a multiplicity of light-emitting diodes closely arrayed on a single substrate for use as a light source such as the light source used to form an image in an optical printer.
2. Description of the Prior Art
A light-emitting diode (LED) array consists of multiple P-N or P-I-N junction LEDs fabricated on a single substrate. An advantage of such a light-emitting diode array is that it can be used to process image information with relative ease, by electrically controlling the discrete diodes of the array. Because of this, light-emitting diode arrays are being improved and applied in a variety of ways.
One example relates to the use of printers as hard-copy data output devices. With the increasing importance of electronic information in today's world, printers need to be able to print faster and at higher densities in order to cope not only with the growing amounts of information, but also with the inclusion of image information such as graphs, drawings and photographs. One way of achieving this is to use light-emitting diode arrays as the light sources in the printers.
Laser printers, which employ a laser light source, and LED printers, in which an LED array forms the light source, are examples of non-impact optical printers. In a laser printer the scanning by the laser beam is effected by mechanical means, such as a rotating polygonal mirror, and a correspondingly complex optical system. An LED printer, on the other hand, only requires a control system for switching the LEDs of the array on and off, and it is therefore possible for LED printers to be smaller and faster than laser printers, as well as more reliable.
FIG. 4 is a cross-sectional illustration of a conventional homojunction type LED array used in an LED printer. For simplicity, in the drawing only two light-emitting diodes (hereinafter also referred to as light-emitting elements) are shown. With reference to the FIG. 4, each light-emitting element is formed by the use of vapor-phase epitaxy (VPE) to deposit an n-GaAsP layer 12 about 15 microns thick on an n-GaAs substrate 10, followed by a SiN.sub.x masking layer 14 and a diffusion of zinc to form zinc diffused regions 16 each about 2.5 microns thick. The light-emitting element is constituted by the P-N junction between the n-GaAsP layer 12 and the zinc diffused regions 16. Next, p-electrode 18 and n-electrode 20 are formed, followed by an antireflection SiN.sub.x layer 22. This SiN.sub.x layer 22 is then removed from the non-light-emitting element portions to form a p-electrode 18 bonding pad.
Two problems arising when such a light-emitting diode array is used in a printer, and which are not encountered when individual LEDs are used, are crosstalk between adjacent light-emitting elements and variation in characteristics from element to element. Owing to internal light absorption and the high refractive index of the zinc diffused regions 16, most of the light is reflected and is therefore not available as external output, resulting in a very low external light output efficiency of no more than several percent.
In the conventional type of LED array shown in FIG. 4, the P-N junction being a homojunction formed by the interface between the n-GaAsP layer 12 and the zinc diffused regions 16 results in a high internal absorption, and in addition no consideration is given to light output loss caused by total reflection.
The AlGaAs single heterojunction type light-emitting diode array shown in FIG. 5 was developed to overcome these drawbacks of the conventional GaAsP light-emitting diode array. With reference to FIG. 5, liquid-phase epitaxy (LPE) is used to form a p-Al.sub.x Ga.sub.1-x As layer 32 (10 microns thick; Zn=5.times.10.sup.17 cm.sup.-3), an n-Aly Ga.sub.l-y As layer 34 (5 microns thick; Te=8.times.10.sup.17 cm.sup.-3) and an n.sup.+ -GaAs layer 36 (0.1 microns thick; Sn=5.times.10.sup.18 cm.sup.-3) on a p-GaAs substrate 30. For emitting light with a wavelength in the region of 720.degree. nm, the aluminum composition is set at x=0.2, y.degree.=.degree.0.5.
Photolithography and chemical etching are used to form mesa-shaped light-emitting regions, with etching expanding about 1 micron into the p-Al.sub.x Ga.sub.1-x As layer 32. Following this formation of mesa-shaped light-emitting regions, the n-electrode 38 and p-electrode 40 are then formed by deposition, and the necessary portions of the n-electrode 38 and the n.sup.+ -GaAs layer 36 is removed by photolithography and wet etching. To complete the fabrication of the heterojunction light-emitting diode array, plasma-CVD is then used to form an antireflection SiN.sub.x layer 42.
Structurally, this is an array of conventional high-luminance LEDS. In this arrangement the effect of the heterojunction is to improve the injection efficiency, while at the same time energy attenuation caused by internal absorption is avoided by using the n-Al.sub.y Ga.sub.l-y As layer 34 which is transparent to the light emitted by the light-emitting p-Al.sub.x Ga.sub.1-x As layer 32, producing an external light output efficiency that is several times higher than that achievable with the homojunction light-emitting diode array of FIG. 4.
However, there are still problems with this conventional LED array arrangement. In particular, as described above, the very high refractive index of the light-emitting regions means that most of the light is reflected and therefore remains in the emission regions, keeping the external light output efficiency to a very low level. This is an inherent problem in a surface type light-emitting diode array.
An object of the present invention is therefore to provide a light-emitting diode array that offers an improved external light output efficiency without loss of reliability or reproducibility.